In a wireless communication system which uses multiple carriers, such as an orthogonal frequency division multiple access (OFDMA) or a single carrier-frequency division multiple access (SC-FDMA), radio resources are a set of continuous sub-carriers and are defined by a time-frequency region on a two-dimensional sphere. A time-frequency region is a rectangular form sectioned by time and sub-carrier coordinates. In other words, one time-frequency region could be a rectangular form sectioned by at least one symbol on a time axis and a plurality of sub-carriers on a frequency axis. Such a time-frequency region can be allocated to an uplink for a specific user equipment (UE), or a base station can transmit the time-frequency region to a specific user equipment in a downlink. In order to define such a time-frequency region on the two-dimensional sphere, the number of OFDM symbols and the number of continuous sub-carriers starting from a point having an offset from a reference point should be given.
An evolved universal mobile telecommunications system (E-UMTS) which is currently being discussed uses 10 ms radio frame comprising 10 sub-frames. Namely, one sub-frame includes two continuous slots. One slot has a length of 0.5 ms. Also, one sub-frame comprises a plurality of OFDM symbols, and a part (for example, first symbol) of the plurality of OFDM symbols can be used for transmission of L1/L2 control information.
FIG. 1 illustrates an example of a structure of physical channels used in the E-UMTS. In FIG. 1, one sub-frame comprises an L1/L2 control information transmission region (hatching part) and a data transmission region (non-hatching part).
FIG. 2 illustrates a general method of transmitting data in the E-UMTS. In the E-UMTS, a hybrid auto repeat request (HARQ) scheme, which is one of data retransmission schemes, is used to improve throughput, thereby enabling desirable communication.
Referring to FIG. 2, the base station transmits downlink scheduling information (hereinafter, referred to as ‘DL scheduling information’) through DL L1/L2 control channel, for example, a physical downlink control channel (PDCCH), to transmit data to a user equipment in accordance with the HARQ scheme. The DL scheduling information includes user equipment identifier (UE ID) or group identifier (group ID) of user equipments, location and duration (resource assignment and duration of assignment) information of radio resources allocated for transmission of downlink data, modulation mode, payload size, transmission parameters such as MIMO related information, HARQ process information, redundancy version, and new data indicator.
In order to notify that DL scheduling information is transmitted through the PDCCH for what user equipment, the user equipment identifier (or group identifier), for example, a radio network temporary identifier (RNTI) is transmitted. The RNTI can be classified into a dedicated RNTI and a common RNTI. The dedicated RNTI is used for data transmission and reception to and from a user equipment of which information is registered with a base station. The common RNTI is used if communication is performed with user equipments, which are not allocated with dedicated RNTI as their information is not registered with the base station. Alternatively, the common RNTI is used for transmission and reception of information used commonly for a plurality of user equipments, such as system information. For example, examples of the common RNTI include RA-RNTI and T-C-RNTI, which are used during a random access procedure through a random access channel (RACH). The user equipment identifier or group identifier can be transmitted in a type of CRC masking in DL scheduling information transmitted through the PDCCH.
User equipments located in a specific cell monitor the PDCCH through the L1/L2 control channel using their RNTI information, and receive DL scheduling information through the corresponding PDCCH if they successfully perform CRC decoding through their RNTI. The user equipments receive downlink data transmitted thereto through a physical downlink shared channel (PDSCH) indicated by the received DL scheduling information.
As described above, in order to efficiently use limited radio resources in the wireless communication system, uplink scheduling and downlink scheduling are performed. Particularly, in the system which uses multiple carriers such as OFDMA or SC-FDMA, since a radio resource block formed by a specific time zone and a specific frequency band can be used by only one user equipment, scheduling, which determines how many radio resources are allocated to each user equipment and also determines when the radio resources are allocated to each user equipment, is very important.
For scheduling, the user equipment can perform a buffer status report (BSR) and a channel resource request. The user equipment can allow a network to efficiently perform scheduling by notifying the network of data stored in its buffer, through the buffer status report. The network can perform proper scheduling by identifying what user equipment needs how many radio resources, using the buffer status report. Meanwhile, the user equipment can actively request the network to allocate radio resources.